feat(arr): token bucket table for subtitle budgets

One row per provider/translator per day; try_spend is a single
atomic upsert so concurrent reconcile closes can't both slip a spend
past the daily cap. No allowance configured reads as unlimited.

crates/arr-db/migrations/0027_subtitle_budgets.sql

@@ -0,0 +1,19 @@
+-- #197. Token bucket state for §15's provider and translator budgets. The
+-- allowance itself is `subtitle_settings.provider_daily_budgets` /
+-- `translator_daily_budgets` (#198); this table only tracks what has already
+-- been spent today, so a bucket with no allowance configured reads as
+-- unlimited rather than zero (0025's own comment on that column).
+--
+-- One row per (kind, name, day); a new day is a fresh row rather than a
+-- reset column, so the refill is "today has no row yet" and needs no cron.
+
+CREATE TABLE subtitle_budget_spend (
+    kind  TEXT NOT NULL CHECK (kind IN ('provider', 'translator')),
+    name  TEXT NOT NULL,
+    -- UTC calendar day, `YYYY-MM-DD`.
+    day   TEXT NOT NULL,
+    -- A provider unit is one download; a translator unit is one character of
+    -- source text sent (DESIGN.md §15: translators bill per character).
+    spent INTEGER NOT NULL DEFAULT 0 CHECK (spent >= 0),
+    PRIMARY KEY (kind, name, day)
+) STRICT;